1. Field of the Invention
The invention relates to a two stage injection molding machine having a pressure sensor device capable of detecting pressures during injection, packing and metering processes so as to implement a feedback control for a two stage injection molding machine.
2. Description of the Related Art
As demand for precision and quality of the injection molding is constantly increased, it becomes more important to control the injection speed, precision, and injection pressure for a two-stage injection molding machine used to fabricate small-scale products. However, the typical injection molding machine adopts a servomotor that merely implements a feedback control in terms of speed and position, without function for detecting pressure values during the injection, packing, and metering processes or function for implementing pressure feedback control, posing a huge limitation for the high precision injection.
Currently, the most common improvement from the conventional injection molding machine involves a loadcell for instantly detecting the force subjected to the screw rod of the injection molding machine. By comparative calculation, the difference between the actual force and preset force is compensated to achieve injection control, packing control, and back pressure metering control. FIG. 5 illustrates a injection rod construction for screw rotating and advancing device for injection molding machine as disclosed in an U.S. Pat. No. 5,421,712 to Laing et al., where the loadcell 40 installed in an injection movable platen 42 sense the axial force change and input the change to a control system, so that the control system correct the output force of the injection servomotor according to the axial force error, so as to correctly control molten pressure for achieving injection pressure control for the closed circuit.
However, for such conventional improvement involving an overly complicated mechanism design, the force is not correctly and thoroughly transmitted through the loadcell 40. Therefore, the injection pressure, packing pressure and metering back pressure sensed by the loadcell 40 are not the actual pressures experienced by the plastic injection screw rod 41. Meanwhile, when the injection and metering processes are performed, the friction force generated between the injection movable platen 42 and the injection guide rod 43 also vary depending on speed of the movement for injection movable platen 42, degree of lubrication, and environmental factors, while these uncertainties also have influence on the injection force. So, the friction force has to be compensated in order to achieve the high precision injection. Yet, according to the injection rod in the above mentioned patent disclosure, the injection force and friction force sensed by the loadcell 40 are all distorted, leading to a lack of feedback signal required for compensating the injection force. Consequently, it is difficult to achieve control of the injection pressure, packing pressure, and metering back pressure required for high speed precision injection molding.
Thus, the problem to be solved herein is to provide a pressure sensor device adapted to the two-stage injection molding machine, for thoroughly sensing the force and pressure data required during the injection, packing, and metering processes with a simple mechanism of the pressure sensor device such that a closed circuit feedback control is implemented, and the conventional problems such as signal distortion and delayed response are resolved as a result.